As LIGO continues its third observing run, the added gravitational wave (GW) events provide the promise of further tests of general relativity (GR). In the future the early inspiral of a select number of stellar mass black-hole binaries, like GW150914, will be detectable in frequency bands as low as 10-100 mHz by future space-based detectors. Multi-band GW astronomy is a promising avenue for improved tests of GR, both in the weak-field regime and its propagation, constraints on eccentricity, and enhanced parameter estimation techniques, among other possibilities. Space-based detectors will also predict, sometimes with years of forewarning, the time with which two black holes will merge, which will act as a forewarning to potential EM counterparts and as an unprecedented opportunity to actively configure ground-based detectors to optimize for specific features of a GW signal. This talk will discuss improvements in tests of GR, added benefits of multi-band GW astronomy, and discuss active interferometric techniques to target specific features of a GW signal and methods to optimize advanced (LIGO and Virgo) and third generation (e.g., Cosmic Explorer and Einstein Telescope) detectors to enhance black-hole spectroscopy.